Indirect Frequency Synthesis
From the block diagram and control theory the following noise transfer functions may be derived
- Phase detector and op amp noise have low pass transfer functions with a DC gain of N/Kf
- Reference noise has a low pass transfer function with DC gain of N
- Noise sources after the loop filter have bandpass noise-transfer functions
- VCO noise has a high pass transfer function with unity gain at high freq.
By balancing these contributors, phase noise and spurious can be optimized
- Only moderate steps sizes are achievable
Settling time is dominated by the loop natural frequency and any pre-tuning circuitry settling (usually slow)